1. Field of the Invention
The invention concerns an improved synthesis of a manganese complex useful as a bleach catalyst.
2. The Related Art
Peroxide bleaching agents for use in laundering have been known for many years. Such agents are effective in removing stains, such as tea, fruit and wine stains, from clothing at or near boiling temperatures. The efficacy of peroxide bleaching agents diminishes sharply at temperatures below 60.degree. C.
It is known that many transition metal ions catalyze the decomposition of H.sub.2 O.sub.2 and H.sub.2 O.sub.2 -liberating percompounds, such as sodium perborate. It has also been suggested that transition metal salts together with a chelating agent be employed to activate peroxide compounds to render them usable for satisfactory bleaching at lower temperatures. Not all combinations of transition metals with chelating agents are suitable for improving the bleaching performance of peroxide compound bleaches. Many combinations indeed show no effect, or even a worsening effect, on the bleaching performance. A recent advance in this technology was described in copending U.S. Patent application No. 07/798,396, filed Nov. 26, 1991. Therein was reported a class of highly active bleaching catalysts in the form of a mononuclear manganese complex having the general formula: ##STR2## especially the species: EQU Mn(MeTACN)(OCH.sub.3).sub.3 (PF.sub.6).
Related dinuclear manganese complexes such as the species Mn(MeTACN).sub.2 (.mu.-O).sub.3 (PF.sub.6).sub.2 .multidot.H.sub.2 O, have been synthesized and described by K. Wieghardt in the "Journal of the American Chemical Society", 1988, Vol. 110, No. 22, page 7398, as well as in the "Journal of the Chemical Society--Chemical Communications", 1988, page 1145.
The synthesis route for dinuclear complexes as described in the above art involves the reaction in aqueous medium of a manganese (III)-compound, e.g. Mn (III)-triacetate, with a proper nitrogen-containing ligand, e.g. 1,4,7-trimethyl-1,4,7triazacyclononane, using an ethanol/water mixture as the solvent. A drawback of the aforementioned process is that only low yields of the dinuclear Mn (III)-complex can be achieved. Another problem associated with the process of the art is that, owing to the slow crystallization of the product, long reaction times are necessary. Still another problem is that besides crystallization of the desired product, decomposition also seems to occur, yielding manganese dioxide which contaminates the product. Therefore, a purification process is required when the product is to be converted into the dinuclear Mn (IV)-complex.
More recently there was reported in co-pending U.S. Patent application Ser. No. 07/798,477 filed Nov. 26, 1991, a process for the preparation of dinuclear manganese complex catalysts wherein a four-step procedure is outlined. Therein a manganese II salt and a ligand L are reacted to form a manganese coordinated substance. In a second and third step, the substance is oxidized and then basified to a pH of at least 10.5, respectively. The fourth step requires contacting the basified reaction mixture with a further oxidizing agent so as to form the final manganese complex catalyst. Yields in the 60% range are thereby achieved.
During investigation of synthesis routes to the dinuclear manganese complex, it was discovered that the use of dry methanol would produce the mononuclear complex whereas addition of water would result in formation of the dinuclear complex. Unfortunately, this route to the mononuclear complex provided yields below 30%, most often below 20%. Improvements in yield and reduction in processing costs would, therefore, be highly desirable.
Accordingly, it is an object of the present invention to provide an improved method for the preparation of manganese (IV) mononuclear complexes.
A more specific object of the present invention is to provide an improved method for preparing manganese complexes of high purity in high yields, which can be converted into the corresponding mononuclear manganese (IV)-complexes by oxidation.
These and other objects of the present invention will become more readily apparent from the detailed description and examples given hereafter.